In present Voice over Internet Protocol (VoIP) networks such as the Internet Protocol Multimedia Subsystem (IMS) telecommunications networks commonly the function of User Agent (UA), sometimes also referred to as client, is deployed in a user equipment such as a Personal Computer (PC) or mobile telephone. In an IMS network the functions of Proxy Call Session Control Function (P-CSCF), Interrogating Call Session Control Function (I-CSCF) and Serving Call Session Control Function (S-CSCF) are commonly deployed in network nodes, inside a secure environment, herein also referred to as “intranet”, of the IMS-provider. Hence, network nodes inside the “intranet” of the IMS-provider are supposed to work inside a secure environment.
The function of Application Server (AS), e.g. Session Initiation Protocol Application Server (SIP-AS), is commonly deployed in network nodes, inside the secure environment of the IMS-provider. It is noted that the SIP-AS can belong to a 3rd party, i.e. the Internet Protocol Multimedia Service Control (ISC) interface (between an S-CSSF and the SIP-AS) may be an external interface. The use of such external interface requires agreements between the IMS-provider and the 3rd party.